In case of manufacturing electronic devices on a substrate, e.g., a wafer through a plurality of steps, a desired pattern including, e.g., trenches or holes is formed on the wafer by performing a film formation process or etching process on the wafer in each step. At this time, a reaction product or unintended foreign substance may be generated in the film formation process or etching process and attached onto the wafer. Since the foreign materials or the like on the wafer adversely affects the processing of the next step, it is necessary to remove it as much as possible.
Conventionally, as a method of removing a foreign material on the wafer, a method of washing the foreign material by immersing the wafer in a chemical tank, or a method of washing the foreign material by spraying pure water or liquid chemical to the wafer has been used. In these methods, since the pure water or liquid chemical remains on the wafer after cleaning of the wafer, the wafer is dried by spin drying or the like.
However, during drying of the wafer, as shown in FIG. 4A, for example, if a liquid chemical 43 remains in trenches 41 and 42, a gas-liquid interfacial tension F is generated at the surface of the liquid chemical 43, and the gas-liquid interfacial tension F acts on convex portions 44a to 44c of the pattern. Thus, as shown in FIG. 4B, the convex portion 44a or 44c of the pattern may collapse.
In order to prevent the collapse of the convex portion of the pattern, a dry cleaning method for cleaning the wafer without the use of pure water and liquid chemical is suitable. As a dry cleaning method, a method of evaporating the foreign material by irradiating a laser beam on the wafer, or a method of physically removing the foreign material by sputtering using a plasma has been known. However, when the laser beam is irradiated on the wafer, the film formed on the wafer may be altered. Further, since the plasma has high energy, not only the foreign material but also the pattern may be cut by sputtering.
Therefore, recently, as a dry cleaning method in which the energy applied to the wafer is not very high, a method of using a gas cluster ion beam (GCIB) has been developed (see, e.g., Patent document 1). The GCIB is a method of forming a cluster of gas molecules by spraying a gas toward the vacuum atmosphere, ionizing the cluster, and allowing the ionized cluster to collide with the wafer by applying a bias voltage to the wafer. The cluster which has collided with the wafer is decomposed into gas molecules to scatter after granting the kinetic energy to the wafer. In this case, the kinetic energy promotes a chemical reaction between the foreign material and gas molecules on the wafer to generate a reaction product. Then, the reaction product is sublimated, thereby removing the foreign material.